1. Technical Field
The present disclosure relates to methods for operating a Discontinuous Reception, DRX, function at a user equipment, wherein the user equipment is configured with at least one licensed cell and at least one unlicensed cell. The present disclosure also provides the user equipment and base station for performing the methods described herein.
2. Description of the Related Art
Long Term Evolution (LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology are being deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies, 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support for the next decade. The ability to provide high bit rates is a key measure for LTE.
The work item (WI) specification on Long-Term Evolution (LTE) called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM)-based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP) and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA)-based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmit power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques and a highly efficient control signaling structure is achieved in LTE Rel. 8/9.
LTE Architecture
The overall LTE architecture is shown in FIG. 1. The E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNodeBs are interconnected with each other by means of the X2 interface.
The eNodeBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME and to the Serving Gateway (SGW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNodeBs. The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PDN GW). For idle-state user equipments, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the user equipment. It manages and stores user equipment contexts, e.g. parameters of the IP bearer service, or network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle-mode user equipment tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a user equipment at the initial attach and at the time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non-Access Stratum (NAS) signaling terminates at the MME, and it is also responsible for the generation and allocation of temporary identities to user equipments. It checks the authorization of the user equipment to camp on the service provider's Public Land Mobile Network (PLMN) and enforces user equipment roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming user equipments.
Component Carrier Structure in LTE
The downlink component carrier of a 3GPP LTE system is subdivided in the time-frequency domain in so-called subframes. In 3GPP LTE each subframe is divided into two downlink slots as shown in FIG. 2, wherein the first downlink slot comprises the control channel region (PDCCH region) within the first OFDM symbols. Each subframe consists of a given number of OFDM symbols in the time domain (12 or 14 OFDM symbols in 3GPP LTE (Release 8)), wherein each OFDM symbol spans over the entire bandwidth of the component carrier. The OFDM symbols thus each consist of a number of modulation symbols transmitted on respective subcarriers. In LTE, the transmitted signal in each slot is described by a resource grid of NDLRBNRBSC subcarriers and NDLsymb OFDM symbols. NDLRB is the number of resource blocks within the bandwidth. The quantity NDLRB depends on the downlink transmission bandwidth configured in the cell and shall fulfill Nmin,DLRB<=NDLRB<=Nmax,DLRB, where Nmin,DLRB=6 and Nmax,DLRB=110 are respectively the smallest and the largest downlink bandwidths, supported by the current version of the specification. NRBSC is the number of subcarriers within one resource block. For normal cyclic prefix subframe structure, NRBSC=12 and NDLsymb=7.
Assuming a multi-carrier communication system, e.g. employing OFDM, as for example used in 3GPP Long Term Evolution (LTE), the smallest unit of resources that can be assigned by the scheduler is one “resource block”. A physical resource block (PRB) is defined as consecutive OFDM symbols in the time domain (e.g. 7 OFDM symbols) and consecutive subcarriers in the frequency domain as exemplified in FIG. 2 (e.g. 12 subcarriers for a component carrier). In 3GPP LTE (Release 8), a physical resource block thus consists of resource elements, corresponding to one slot in the time domain and 180 kHz in the frequency domain (for further details on the downlink resource grid, see for example 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8)” version 12.5.0, section 6.2, available at http://www.3gpp.org and incorporated herein by reference).
One subframe consists of two slots, so that there are 14 OFDM symbols in a subframe when a so-called “normal” CP (cyclic prefix) is used, and 12 OFDM symbols in a subframe when a so-called “extended” CP is used. For sake of terminology, in the following the time-frequency resources equivalent to the same consecutive subcarriers spanning a full subframe is called a “resource block pair”, or equivalent “RB pair” or “PRB pair”.
The term “component carrier” refers to a combination of several resource blocks in the frequency domain. In future releases of LTE, the term “component carrier” is no longer used; instead, the terminology is changed to “cell”, which refers to a combination of downlink and optionally uplink resources. The linking between the carrier frequency of the downlink resources and the carrier frequency of the uplink resources is indicated in the system information transmitted on the downlink resources.
Similar assumptions for the component carrier structure will apply to later releases too.
Carrier Aggregation in LTE-A for Support of Wider Bandwidth
The frequency spectrum for IMT-Advanced was decided at the World Radio communication Conference 2007 (WRC-07). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved. The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced.
The bandwidth that the LTE-Advanced system is able to support is 100 MHz, while an LTE system can only support 20 MHz. Nowadays, the lack of radio spectrum has become a bottleneck of the development of wireless networks, and as a result it is difficult to find a spectrum band which is wide enough for the LTE-Advanced system. Consequently, it is urgent to find a way to gain a wider radio spectrum band, wherein a possible answer is the carrier aggregation functionality.
In carrier aggregation, two or more component carriers are aggregated in order to support wider transmission bandwidths up to 100 MHz. Several cells in the LTE system are aggregated into one wider channel in the LTE-Advanced system which is wide enough for 100 MHz even though these cells in LTE may be in different frequency bands.
All component carriers can be configured to be LTE Rel. 8/9 compatible, at least when the bandwidth of a component carrier does not exceed the supported bandwidth of an LTE Rel. 8/9 cell. Not all component carriers aggregated by a user equipment may necessarily be Rel. 8/9 compatible. Existing mechanisms (e.g. barring) may be used to avoid Rel-8/9 user equipments to camp on a component carrier.
A user equipment may simultaneously receive or transmit on one or multiple component carriers (corresponding to multiple serving cells) depending on its capabilities. An LTE-A Rel. 10 user equipment with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple serving cells, whereas an LTE Rel. 8/9 user equipment can receive and transmit on a single serving cell only, provided that the structure of the component carrier follows the Rel. 8/9 specifications.
Carrier aggregation is supported for both contiguous and non-contiguous component carriers with each component carrier limited to a maximum of 110 Resource Blocks in the frequency domain (using the 3GPP LTE (Release 8/9) numerology).
It is possible to configure a 3GPP LTE-A (Release 10)-compatible user equipment to aggregate a different number of component carriers originating from the same eNodeB (base station) and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the UE. Conversely, the number of uplink component carriers that can be configured depends on the uplink aggregation capability of the UE. It may currently not be possible to configure a mobile terminal with more uplink component carriers than downlink component carriers. In a typical TDD deployment, the number of component carriers and the bandwidth of each component carrier in uplink and downlink are the same. Component carriers originating from the same eNodeB need not provide the same coverage.
The spacing between center frequencies of contiguously aggregated component carriers shall be a multiple of 300 kHz. This is in order to be compatible with the 100 kHz frequency raster of 3GPP LTE (Release 8/9) and at the same time to preserve orthogonality of the subcarriers with 15 kHz spacing. Depending on the aggregation scenario, the n*300 kHz spacing can be facilitated by insertion of a low number of unused subcarriers between contiguous component carriers.
The nature of the aggregation of multiple carriers is only exposed up to the MAC layer. For both uplink and downlink, there is one HARQ entity required in MAC for each aggregated component carrier. There is (in the absence of SU-MIMO for uplink) at most one transport block per component carrier. A transport block and its potential HARQ retransmissions need to be mapped on the same component carrier.
When carrier aggregation is configured, the mobile terminal only has one RRC connection with the network. At RRC connection establishment/re-establishment, one cell provides the security input (one ECGI, one PCI and one ARFCN) and the non-access stratum mobility information (e.g. TAI) similarly as in LTE Rel. 8/9. After RRC connection establishment/re-establishment, the component carrier corresponding to that cell is referred to as the downlink Primary Cell (PCell). There is always one and only one downlink PCell (DL PCell) and one uplink PCell (UL PCell) configured per user equipment in connected state. Within the configured set of component carriers, other cells are referred to as Secondary Cells (SCells); with carriers of the SCell being the Downlink Secondary Component Carrier (DL SCC) and Uplink Secondary Component Carrier (UL SCC). Maximum five serving cells, including the PCell, can be configured for one UE.
The characteristics of the downlink and uplink PCell are:                For each SCell, the usage of uplink resources by the UE in addition to the downlink ones is configurable (the number of DL SCCs configured is therefore always larger or equal to the number of UL SCCs, and no SCell can be configured for usage of uplink resources only)        The downlink PCell cannot be de-activated, unlike SCells        Re-establishment is triggered when the downlink PCell experiences Rayleigh fading (RLF), not when downlink SCells experience RLF        Non-access stratum information is taken from the downlink PCell        PCell can only be changed with handover procedure (i.e. with security key change and RACH procedure)        PCell is used for transmission of PUCCH        The uplink PCell is used for transmission of Layer 1 uplink control information        From a UE viewpoint, each uplink resource only belongs to one serving cell        
The configuration and reconfiguration, as well as addition and removal, of component carriers can be performed by RRC. Activation and deactivation is done via MAC control elements. At intra-LTE handover, RRC can also add, remove, or reconfigure SCells for usage in the target cell. When adding a new SCell, dedicated RRC signaling is used for sending the system information of the SCell, the information being necessary for transmission/reception (similarly as in Rel-8/9 for handover). Each SCell is configured with a serving cell index, when the SCell is added to one UE; PCell has always the serving cell index 0.
When a user equipment is configured with carrier aggregation, there is at least one pair of uplink and downlink component carriers that is always active. The downlink component carrier of that pair might be also referred to as ‘DL anchor carrier’. Same applies also for the uplink.
When carrier aggregation is configured, a user equipment may be scheduled on multiple component carriers simultaneously, but at most one random access procedure shall be ongoing at any time. Cross-carrier scheduling allows the PDCCH of a component carrier to schedule resources on another component carrier. For this purpose a component carrier identification field is introduced in the respective DCI (Downlink Control Information) formats, called CIF.
A linking, established by RRC signaling, between uplink and downlink component carriers allows identifying the uplink component carrier for which the grant applies when there is no cross-carrier scheduling. The linkage of downlink component carriers to uplink component carrier does not necessarily need to be one to one. In other words, more than one downlink component carrier can link to the same uplink component carrier. At the same time, a downlink component carrier can only link to one uplink component carrier.
Layer 1/Layer 2 Control Signaling
In order to inform the scheduled users about their allocation status, transport format and other transmission-related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length can be a multiple of the subframes. The TTI length may be fixed in a service area for all users, may be different for different users, or may even be dynamic for each user. Generally, the L1/2 control signaling needs only to be transmitted once per TTI. Without loss of generality, the following assumes that a TTI is equivalent to one subframe.
The L1/L2 control signaling is transmitted on the Physical Downlink Control Channel (PDCCH).
A PDCCH carries a message as a Downlink Control Information (DCI), which in most cases includes resource assignments and other control information for a mobile terminal or groups of UEs. In general, several PDCCHs can be transmitted in one subframe.
It should be noted that in 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH.
Furthermore, Release 11 introduced an EPDCCH that fulfills basically the same function as the PDCCH, i.e. conveys L1/L2 control signaling, even though the detailed transmission methods are different from the PDCCH. Further details can be found particularly in the current versions of 3GPP TS 36.211 and 36.213, incorporated herein by reference. Consequently, most items outlined in the background and the embodiments apply to PDCCH as well as EPDCCH, or other means of conveying L1/L2 control signals, unless specifically noted.
Generally, the information sent in the L1/L2 control signaling for assigning uplink or downlink radio resources (particularly LTE(-A) Release 10) can be categorized to the following items:                User identity, indicating the user that is allocated. This is typically included in the checksum by masking the CRC with the user identity;        Resource allocation information, indicating the resources (e.g. Resource Blocks, RBs) on which a user is allocated. Alternatively this information is termed resource block assignment (RBA). Note, that the number of RBs on which a user is allocated can be dynamic;        Carrier indicator, which is used if a control channel transmitted on a first carrier assigns resources that concern a second carrier, i.e. resources on a second carrier or resources related to a second carrier; (cross carrier scheduling);        Modulation and coding scheme that determines the employed modulation scheme and coding rate;        HARQ information, such as a new data indicator (NDI) and/or a redundancy version (RV) that is particularly useful in retransmissions of data packets or parts thereof;        Power control commands to adjust the transmit power of the assigned uplink data or control information transmission;        Reference signal information such as the applied cyclic shift and/or orthogonal cover code index, which are to be employed for transmission or reception of reference signals related to the assignment;        Uplink or downlink assignment index that is used to identify an order of assignments, which is particularly useful in TDD systems;        Hopping information, e.g. an indication whether and how to apply resource hopping in order to increase the frequency diversity;        CSI request, which is used to trigger the transmission of channel state information in an assigned resource; and        Multi-duster information, which is a flag used to indicate and control whether the transmission occurs in a single cluster (contiguous set of RBs) or in multiple clusters (at least two non-contiguous sets of contiguous RBs). Multi-cluster allocation has been introduced by 3GPP LTE-(A) Release 10.        
It is to be noted that the above listing is non-exhaustive, and not all mentioned information items need to be present in each PDCCH transmission depending on the DCI format that is used.
Downlink control information occurs in several formats that differ in overall size and also in the information contained in their fields as mentioned above. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, “Multiplexing and channel coding”, section 5.3.3.1 (current version v12.4.0 available at http://www.3gpp.org and incorporated herein by reference). In addition, for further information regarding the DCI formats and the particular information that is transmitted in the DCI, please refer to the mentioned technical standard or to LTE—The UMTS Long Term Evolution—From Theory to Practice, Edited by Stefanie Sesia, Issam Toufik, Matthew Baker, Chapter 9.3, incorporated herein by reference.                Format 0: DCI Format 0 is used for the transmission of resource grants for the PDSCH, using single-antenna port transmissions in uplink transmission mode 1 or 2.        Format 1: DCI Format 1 is used for the transmission of resource assignments for single codeword PDSCH transmissions (downlink transmission modes 1, 2 and 7).        Format 1A: DCI Format 1A is used for compact signaling of resource assignments for single codeword PDSCH transmissions, and for allocating a dedicated preamble signature to a mobile terminal for contention-free random access (for all transmissions modes).        Format 1B: DCI Format 1B is used for compact signaling of resource assignments for PDSCH transmissions using closed loop precoding with rank-1 transmission (downlink transmission mode 6). The information transmitted is the same as in Format 1A, but with the addition of an indicator of the precoding vector applied for the PDSCH transmission.        Format 1C: DCI Format 1C is used for very compact transmission of PDSCH assignments. When format 1C is used, the PDSCH transmission is constrained to using QPSK modulation. This is used, for example, for signaling paging messages and broadcast system information messages.        Format 1D: DCI Format 1D is used for compact signaling of resource assignments for PDSCH transmission using multi-user MIMO. The information transmitted is the same as in Format 1B, but instead of one of the bits of the precoding vector indicators, there is a single bit to indicate whether a power offset is applied to the data symbols. This feature is needed to show whether or not the transmission power is shared between two UEs. Future versions of LTE may extend this to the case of power sharing between larger numbers of UEs.        Format 2: DCI Format 2 is used for the transmission of resource assignments for PDSCH for closed-loop MIMO operation (transmission mode 4).        Format 2A: DCI Format 2A is used for the transmission of resource assignments for PDSCH for open-loop MIMO operation. The information transmitted is the same as for Format 2, except that if the eNodeB has two transmit antenna ports, there is no precoding information, and for four antenna ports, two bits are used to indicate the transmission rank (transmission mode 3).        Format 2B: Introduced in Release 9 and is used for the transmission of resource assignments for PDSCH for dual-layer beamforming (transmission mode 8).        Format 2C: Introduced in Release 10 and is used for the transmission of resource assignments for PDSCH for closed-loop single-user or multi-user MIMO operation with up to 8 layers (transmission mode 9).        Format 2D: introduced in Release 11 and used for up to 8 layer transmissions; mainly used for COMP (Cooperative Multipoint) (transmission mode 10).        Format 3 and 3A: DCI formats 3 and 3A are used for the transmission of power control commands for PUCCH and PUSCH with 2-bit or 1-bit power adjustments respectively. These DCI formats contain individual power control commands for a group of UEs.        Format 4: DCI format 4 is used for the scheduling of the PUSCH, using closed-loop spatial multiplexing transmissions in uplink transmission mode 2.DRX—Discontinuous Reception        
DRX functionality can be configured for RRC_IDLE, in which case the UE uses either the specific or default DRX value (defaultPagingCycle); the default is broadcasted in the System Information, and can have values of 32, 64, 128 and 256 radio frames. If both specific and default values are available, the shorter value of the two is chosen by the UE. The UE needs to wake up for one paging occasion per DRX cycle, the paging occasion being one subframe. DRX functionality can be also configured for an “RRC_CONNECTED” UE, so that it does not always need to monitor the downlink channels. In order to provide reasonable battery consumption of user equipment, 3GPP LTE (Release 8/9) as well as 3GPP LTE-A (Release 10) provides a concept of discontinuous reception (DRX). Technical Standard TS 36.321, “Evolved Universal Terrestrial Radio Access (E-UTRA); Medium Access Control (MAC) protocol specification” version 12.5.0, chapter 5.7 explains the DRX and is incorporated by reference herein.
The following parameters are available to define the DRX UE behavior; i.e. the On-Duration periods at which the mobile node is active (i.e. in DRX Active Time), and the periods where the mobile node is in DRX (i.e. not in DRX Active Time).                On-duration: duration in downlink subframes, i.e. more in particular in subframes with PDCCH (also referred to as PDCCH subframe), that the user equipment, after waking up from DRX, receives and monitors the PDCCH. It should be noted here that throughout this present disclosure the term “PDCCH” refers to the PDCCH, EPDCCH (in subframes when configured) or, for a relay node with R-PDCCH configured and not suspended, to the R-PDCCH. If the user equipment successfully decodes a PDCCH, the user equipment stays awake/active and starts the inactivity timer; [1-200 subframes; 16 steps: 1-6, 10-60, 80, 100, 200]        DRX inactivity timer: duration in downlink subframes that the user equipment waits to successfully decode a PDCCH, from the last successful decoding of a PDCCH; when the UE fails to decode a PDCCH during this period, it re-enters DRX. The user equipment shall restart the inactivity timer following a single successful decoding of a PDCCH for a first transmission only (i.e. not for retransmissions). [1-2560 subframes; 22 steps, 10 spares: 1-6, 8, 10-60, 80, 100-300, 500, 750, 1280, 1920, 2560]        DRX Retransmission timer: specifies the number of consecutive PDCCH subframes where a downlink retransmission is expected by the UE after the first available retransmission time. [1-33 subframes, 8 steps: 1, 2, 4, 6, 8, 16, 24, 33]        DRX short cycle: specifies the periodic repetition of the on-duration followed by a possible period of inactivity for the short DRX cycle. This parameter is optional. [2-640 subframes; 16 steps: 2, 5, 8, 10, 16, 20, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640]        DRX short cycle timer: specifies the number of consecutive subframes the UE follows the short DRX cycle after the DRX Inactivity Timer has expired. This parameter is optional. [1-16 subframes]        Long DRX Cycle Start offset: specifies the periodic repetition of the on-duration followed by a possible period of inactivity for the DRX long cycle as well as an offset in subframes when on-duration starts (determined by formula defined in TS 36.321 section 5.7); [cycle length 10-2560 subframes; 16 steps: 10, 20, 30, 32, 40, 64, 80, 128, 160, 256, 320, 512, 640, 1024, 1280, 2048, 2560; offset is an integer between [0 -subframe length of chosen cycle]]        
The total duration that the UE is awake is called “Active time” or DRX Active Time. The Active Time e.g. includes the on-duration of the DRX cycle, the time UE is performing continuous reception while the inactivity timer has not expired and the time UE is performing continuous reception while waiting for a downlink retransmission after one HARQ RTT. Similarly, for the uplink the UE is awake (i.e. in DRX Active Time) at subframes where uplink retransmission grants can be received, i.e. every 8 ms after an initial uplink transmission until the maximum number of retransmissions is reached. Based on the above, the minimum Active Time is of fixed length equal to on-duration, and the maximum is variable depending on e.g. the PDCCH activity.
The “DRX period” is the duration of downlink subframes during which a UE can skip reception of downlink channels for battery saving purposes. The operation of DRX gives the mobile terminal the opportunity to deactivate the radio circuits repeatedly (according to the currently active DRX cycle) in order to save power. Whether the UE indeed remains in DRX (i.e. is not active) during the DRX period may be decided by the UE; for example, the UE usually performs inter-frequency measurements which cannot be conducted during the On-Duration, and thus need to be performed at some other time, e.g. during the DRX opportunity time.
The parameterization of the DRX cycle involves a trade-off between battery saving and latency. For example, in case of a web browsing service, it is usually a waste of resources for a UE to continuously receive downlink channels while the user is reading a downloaded web page. On the one hand, a long DRX period is beneficial for lengthening the UE's battery life. On the other hand, a short DRX period is better for faster response when data transfer is resumed—for example when a user requests another web page.
To meet these conflicting requirements, two DRX cycles—a short cycle and a long cycle—can be configured for each UE; the short DRX cycle is optional, i.e. only the long DRX cycle could be used. The transition between the short DRX cycle, the long DRX cycle and continuous reception is controlled either by a timer or by explicit commands from the eNodeB. In some sense, the short DRX cycle can be considered as a confirmation period in case a late packet arrives, before the UE enters the long DRX cycle. If data arrives at the eNodeB while the UE is in the short DRX cycle, the data is scheduled for transmission at the next on-duration time, and the UE then resumes continuous reception. On the other hand, if no data arrives at the eNodeB during the short DRX cycle, the UE enters the long DRX cycle, assuming that the packet activity is finished for the time being.
During the Active Time, the UE monitors PDCCH, reports SRS (Sounding Reference Signal) as configured and reports CQI (Channel Quality Information)/PMI (Preceding Matrix Indicator)/RI (Rank Indicator)/PTI (Precoder Type Indication) on PUCCH. When UE is not in Active time, type-0-triggered SRS and CQI/PMI/RI/PTI on PUCCH may not be reported. If CQI masking is set up for the UE, the reporting of CQI/PMI/RI/PTI on PUCCH is limited to the On-Duration subframes.
Available DRX values are controlled by the network and start from non-DRX up to x seconds. Value x may be any value as long as the paging DRX is used in RRC_IDLE. Measurement requirements and reporting criteria can differ according to the length of the DRX interval, i.e. long DRX intervals may have more relaxed requirements (for more details see further below). When DRX is configured, periodic DCI reports can only be sent by the UE during “active-time”. RRC can further restrict periodic CQI reports so that they are only sent during the on-duration.
FIG. 3 discloses an example of an DRX operation. The UE checks for scheduling messages (e.g. indicated by its C-RNTI, cell radio network temporary identity, on the PDCCH) during the “on-duration” period, which is the same for the long DRX cycle and the short DRX cycle. When a scheduling message is received during an “on-duration period”, the UE starts an “inactivity timer” and keeps monitoring the PDCCH in every subframe while the Inactivity Timer is running. During this period, the UE can be regarded as being in a “continuous reception mode”. Whenever a scheduling message is received while the Inactivity Timer is running, the UE restarts the Inactivity Timer, and when it expires, the UE moves into a short DRX cycle and starts a “short DRX cycle timer” (assuming a short DRX cycle is configured). When the short DRX cycle timer expires, the UE moves into a long DRX cycle. The short DRX cycle may also be initiated by means of a DRX MAC Control Element, which the eNB can send at any time to put the UE immediately into a DRX cycle, i.e. the short DRX cycle (if so configured) or long DRX cycle (in case the short DRX cycle is not configured).
In 3GPP Release 11 a new DRX MAC control element, called Long DRX Command MAC CE, was introduced which allows the eNB to order the UE to go immediately into the Long DRX cycle—without cycling first through the short DRX cycle—for the case that the short DRX cycle is configured.
In addition to this DRX behavior, a ‘HARQ Round Trip Time (RTT) timer’ is defined with the aim of allowing the UE to sleep during the HARQ RTT. When decoding of a downlink transport block for one HARQ process fails, the UE can assume that the next retransmission of the transport block will occur after at least ‘HARQ RTT’ subframes. While the HARQ RTT timer is running, the UE does not need to monitor the PDCCH. At the expiry of the HARQ RTT timer, the UE resumes reception of the PDCCH as normal.
The above-mentioned DRX-related timers, like the DRX-Inactivity timer, the HARQ RTT timer, the DRX retransmission timer, and the Short DRX cycle timer, are started and stopped by events such as the reception of a PDCCH grant or a MAC control element (DRX MAC CE). Hence, the DRX status (active time or non-active time) of the UE can change from subframe to subframe and thus is not always predictable by the mobile node.
At present, for carrier aggregation, a common DRX operation is applied to all configured and activated serving cells of a UE; this is also referred to as UE-specific DRX. Essentially, the Active Time is the same for all cells. Hence, the UE is monitoring PDCCH of all DL Cells in the same subframe. DRX-related timers and parameters are configured per UE, not per cell, such that there is only one DRX cycle per user equipment. All aggregated component carriers follow this “common” DRX pattern.
LTE on Unlicensed Bands—Licensed-Assisted Access LAA
In September 2014, 3GPP initiated a new study item on LTE operation on unlicensed spectrum. The reason for extending LTE to unlicensed bands is the ever-growing demand for wireless broadband data in conjunction with the limited amount of licensed bands. The unlicensed spectrum therefore is more and more considered by cellular operators as a complementary tool to augment their service offering. The advantage of LTE in unlicensed bands compared to relying on other radio access technologies (RAT) such as Wi-Fi is that complementing the LTE platform with unlicensed spectrum access enables operators and vendors to leverage the existing or planned investments in LTE/EPC hardware in the radio and core network.
However, it has to be taken into account that unlicensed spectrum access can never match the qualities of licensed spectrum access due to the inevitable coexistence with other radio access technologies (RATs) in the unlicensed spectrum. LTE operation on unlicensed bands will therefore at least in the beginning be considered a complement to LTE on licensed spectrum rather than as stand-alone operation on unlicensed spectrum. Based on this assumption, 3GPP established the term Licensed Assisted Access (LAA) for the LTE operation on unlicensed bands in conjunction with at least one licensed band. Future stand-alone operation of LTE on unlicensed spectrum without relying on LAA however shall not be excluded.
The currently-intended general LAA approach at 3GPP is to make use of the already specified Rel-12 carrier aggregation (CA) framework as much as possible, where the CA framework configuration as explained before comprises a so-called primary cell (PCell) carrier and one or more secondary cell (SCell) carriers. CA supports in general both self-scheduling of cells (scheduling information and user data are transmitted on the same component carrier) and cross-carrier scheduling between cells (scheduling information in terms of PDCCH/EPDCCH and user data in terms of PDSCH/PUSCH are transmitted on different component carriers). This includes that a common DRX scheme is used for LAA, particularly if it does not result in a need for very short DRX cycles/very long Active Times. As with carrier aggregation mentioned above, “common DRX” scheme in this respect means that the UE operates the same DRX for all aggregated and activated cells, including unlicensed and licensed cells. Consequently, the Active Time is the same for all serving cells, e.g. UE is monitoring PDCCH of all downlink serving cells in the same subframe; the DRX-related timers and parameters are configured per UE.
A very basic scenario is illustrated in FIG. 4, with a licensed PCell, licensed SCell 1, and various unlicensed SCells 2, 3, and 4 (exemplarily depicted as small cells). The transmission/reception network nodes of unlicensed SCells 2, 3, and 4 could be remote radio heads managed by the eNB or could be nodes that are attached to the network but not managed by the eNB. For simplicity, the connection of these nodes to the eNB or to the network is not explicitly shown in the figure.
At present, the basic approach envisioned at 3GPP is that the PCell will be operated on a licensed band while one or more SCells will be operated on unlicensed bands. The benefit of this strategy is that the PCell can be used for reliable transmission of control messages and user data with high quality of service (QoS) demands, such as for example voice and video, while an SCell on unlicensed spectrum might yield, depending on the scenario, to some extent significant QoS reduction due to inevitable coexistence with other RATs.
It has been agreed during RAN1#78bis that the LAA investigation at 3GPP will focus on unlicensed bands at 5 GHz. One of the most critical issues is therefore the coexistence with Wi-Fi (IEEE 802.11) systems operating at these unlicensed bands. In order to support fair coexistence between LTE and other technologies such as Wi-Fi as well to guarantee fairness between different LTE operators in the same unlicensed band, the channel access of LTE for unlicensed bands has to abide by certain sets of regulatory rules which depend on a region and particular frequency band; a comprehensive description of the regulatory requirements for all regions for operation on unlicensed bands at 5 GHz is given in R1-144348, “Regulatory Requirements for Unlicensed Spectrum”, Alcatel-Lucent et al., RAN1#78bis, September 2014, incorporated herein by reference. Depending on a region and band, regulatory requirements that have to be taken into account when designing LAA procedures comprise Dynamic Frequency Selection (DFS), Transmit Power Control (TPC), Listen Before Talk (LBT) and discontinuous transmission with limited maximum transmission duration. The intention of 3GPP is to target a single global framework for LAA which basically means that all requirements for different regions and bands at 5 GHz have to be taken into account for the system design.
The listen-before-talk (LBT) procedure is defined as a mechanism by which an equipment applies a clear channel assessment (CCA) check before using the channel. The CCA utilizes at least energy detection to determine the presence or absence of other signals on a channel in order to determine if a channel is occupied or clear, respectively. European and Japanese regulations mandate the usage of LBT in the unlicensed bands. Apart from regulatory requirements, carrier sensing via LBT is one way for fair sharing of the unlicensed spectrum, and hence it is considered to be a vital feature for fair and friendly operation in the unlicensed spectrum in a single global solution framework.
In an unlicensed spectrum, channel availability cannot always be guaranteed. In addition, certain regions such as Europe and Japan prohibit continuous transmission and impose limits on the maximum duration of a transmission burst in the unlicensed spectrum. Hence, discontinuous transmission with limited maximum transmission duration is a required functionality for LAA.
DFS is required for certain regions and bands in order to detect interference from radar systems and to avoid co-channel operation with these systems. The intention is furthermore to achieve a near-uniform loading of the spectrum. The DFS operation and corresponding requirements are associated with a master-slave principle. The master shall detect radar interference, can however rely on another device, associated with the master, to implement radar detection.
The operation on unlicensed bands at 5-GHz is in most regions limited to rather low transmit power levels compared to the operation on licensed bands which results in small coverage areas. Even if the licensed and unlicensed carriers were to be transmitted with identical power, usually the unlicensed carrier in the 5 GHz band would be expected to support a smaller coverage area than a licensed cell in the 2 GHz band due to increased path loss and shadowing effects for the signal. A further requirement for certain regions and bands is the use of TPC in order to reduce the average level of interference caused for other devices operating on the same unlicensed band.
Detailed information can be found in the harmonized European standard ETSI EN 301 893, current version 1.8.0, incorporated herein by reference.
Following this European regulation regarding LBT, devices have to perform a Clear Channel Assessment (CCA) before occupying the radio channel with a data transmission. It is only allowed to initiate a transmission on the unlicensed channel after detecting the channel as free based e.g. on energy detection. In particular, the equipment has to observe the channel for a certain minimum time (e.g. for Europe 20 μs, see ETSI 301 893, under clause 4.8.3) during the CCA. The channel is considered occupied if the detected energy level exceeds a configured CCA threshold (e.g. for Europe, −73 dBm/MHz, see ETSI 301 893, under clause 4.8.3), and conversely is considered to be free if the detected power level is below the configured CCA threshold. If the channel is determined as being occupied, it shall not transmit on that channel during the next Fixed Frame Period. If the channel is classified as free, the equipment is allowed to transmit immediately. The maximum transmit duration is restricted in order to facilitate fair resource sharing with other devices operating on the same band.
The energy detection for the CCA is performed over the whole channel bandwidth (e.g. 20 MHz in unlicensed bands at 5 GHz), which means that the reception power levels of all subcarriers of an LTE OFDM symbol within that channel contribute to the evaluated energy level at the device that performed the CCA.
Furthermore, the total time during which an equipment has transmissions on a given carrier without re-evaluating the availability of that carrier (i.e. LBT/CCA) is defined as the Channel Occupancy Time (see ETSI 301 893, under clause 4.8.3.1). The Channel Occupancy Time shall be in the range of 1 ms to 10 ms, where the maximum Channel Occupancy Time could be e.g. 4 ms as currently defined for Europe. Furthermore, there is a minimum Idle time the UE is not allowed to transmit after a transmission on the unlicensed cell, the minimum Idle time being at least 5% of the Channel Occupancy Time. Towards the end of the Idle Period, the UE can perform a new CCA, and so on. This transmission behavior s schematically illustrated in FIG. 5, the figure being taken from ETSI EN 301 893 (there FIG. 2: “Example of timing for Frame Based Equipment”).
Considering the different regulatory requirements, it is apparent that the LTE specification for operation in unlicensed bands will require several changes compared to the current Rel-12 specification that is limited to licensed band operation. The currently-defined DRX operation can have several disadvantages when being applied by UEs having an aggregated unlicensed cell.